fs: `walkSync` returns a type without iterator helpers

[greentore]

Describe the bug
walkSync returns IterableIterator<WalkEntry>, which doesn't implement iterator helper methods. However it's implemented as a generator function so it has them at the runtime.

Steps to Reproduce

const a = walkSync(".");
const b = a.map(v => v.name); // Property 'map' does not exist on type 'IterableIterator<WalkEntry>'.

Environment

• OS: Windows 10
• deno version: deno 2.7.11
• std version: 1.0.23